Although lithium ion secondary batteries have been conventionally used as a main power-storage device in fields such as mobile phones, notebook personal computers, and electric vehicles; much smaller and lighter secondary batteries are in demand in recent years. As a result, research is actively conducted for a next-generation secondary battery having a higher energy density than that of lithium ion secondary batteries, and various experiments are underway. In particular, a lithium-air cell, which uses oxygen in the air as a positive electrode active material and uses lithium metal as a negative electrode active material, theoretically has an extremely large discharge capacity since the lithium-air cell does not need to have oxygen, which is the positive electrode active material, disposed inside the cell. Thus, the lithium-air cell is gathering high expectation as a post lithium ion secondary battery for applications such as next-generation electric vehicles and power storage systems for solar/wind power generation facilities. However, reversible and stable charge-discharge is currently difficult to conduct, and the prospect of having the lithium-air cell put to practical use is nowhere in sight.
The lithium-air cell functions through a mechanism in which oxygen reduction (discharging) and oxygen generation (charging) occur at a positive electrode, and lithium dissolution (discharging) and lithium deposition (charging) occur at a negative electrode to enable charging-discharging. Specifically, an oxygen reduction reaction as shown in the following progresses at the positive electrode.

When a solvent in an electrolytic solution is unstable with respect to radicals, a degradative reaction of the solvent is known to occur without the generation of Li2O2 (Non-Patent Art 1). Thus, for a reversible positive electrode reaction, a solvent that is stable with respect to the radicals has to be used. However, since a carbonate based solvent (propylene carbonate etc.) used in conventional lithium ion batteries reacts with oxygen radicals, the reaction at the positive electrode is reported to be irreversible (Non-Patent Art 2 and 3).
Thus, the selection of the electrolytic solution becomes very important for the reversible progression of the positive electrode reaction for usage in secondary batteries. As solvents capable of harboring such a reversible positive electrode reaction, 1,2-dimethoxyethane, acetonitrile, and ionic liquids etc., have been studied (Non-Patent Art 2, 3, and 4). However, even if the problem of reversibility on the positive electrode side were to be solved by the usage of such solvents, there is an unsolved problem on the negative electrode side, i.e., metal lithium being deposited in a branch-like (dendrite) manner during charging when lithium metal is used as the negative electrode, and the dendrite growing and ultimately reaching the positive electrode through repeated charge-discharge to form a short circuit. Furthermore, when acetonitrile is used as a solvent, the problem is that acetonitrile cannot be used in an air cell of which negative electrode is lithium metal since acetonitrile is highly reactive against lithium metal. Thus, the barrier that currently prevents a lithium-air cell using lithium metal as the negative electrode from being put to practical use is extremely high under present circumstances.
On the other hand, in lithium ion batteries, such a problem of dendrite deposition at the negative electrode is known to be solved by using a carbon material such as graphite or the like as the negative electrode active material. However, reversible insertion/extraction of lithium ions with respect to the negative electrode carbon material have been generally considered not possible with the electrolytic solution solvents, such as 1,2-dimethoxyethane, acetonitrile, and ionic liquids, required for the reversible reaction at the positive electrode using oxygen as the positive electrode active material. Thus, negative electrodes formed of a carbon material in an air cell have been considered as not being a candidate for research. In more detail, a reason for that is because insertion/extraction reactions of lithium ions at a negative electrode formed of a carbon material were generally considered only achievable in the presence of a carbonate based solvent. Additional reasons include undesirable phenomena considered to be caused by the properties of the solvents such as: acetonitrile being not able to withstand electric potential for lithium ion insertion due to being susceptible against reduction; 1,2-dimethoxyethane being inserted to the negative electrode together with lithium ions; and ionic liquids similarly causing insertion of cation species to the negative electrode.
Because of the background described above, building an air cell which solves the problem regarding dendrite deposition at the negative electrode while using a solvent sustainable of a reversible positive electrode reaction has been strongly demanded.